Drop-on-demand (DOD) liquid emission devices have been known as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and Stemme in U.S. Pat. No. 3,747,120. A currently popular form of ink jet printing, thermal ink jet (or “bubble jet”), uses electrically resistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara, et al., in U.S. Pat. No. 4,296,421. Although the majority of the market for drop ejection devices is for the printing of inks, other markets are emerging such as ejection of polymers, conductive inks, or drug delivery.
In the past, print head fabrication involved the lamination of a nozzle plate onto the printhead. With this method alignment of the nozzle to the heater is difficult. Also the thickness of the nozzle plate is limited to above a certain thickness. Recently monolithic print heads have been developed through print head manufacturing processes which use photo imaging techniques. The components are constructed on a substrate by selectively adding and subtracting layers of various materials.
Ohkuma et al., in U.S. Pat. No. 5,478,606 discloses a method of monolithically fabricating an ink flow path and chamber with a nozzle plate. FIG. 1 shows the prior art with a substrate 1 containing electrothermal elements 2, and an ink feed port 3. A photo-patternable resin 5 is formed on top of a dissoluble resin that defines the ink flow path including chamber 4. The dissoluble resin is subsequently removed to form the ink flow path and chamber.
In this method of forming ink flow path and chamber; the adjoining of the substrate 1 containing the electrothermal elements 2 and the ink flow path-forming member relies on the adhesion force of the resin 5 constituting the flow path-forming member. In the ink jet head, the flow path and chamber is constantly filled with ink in the normal state of use so that the periphery of the adjoining portion between the substrate and the flow path-forming member is in constant contact with the ink. Therefore, if the adjoining is achieved by the adhesion force only of the resin material, constituting the flow path-forming member, this adhesion can be deteriorated by the influence of the ink. The adhesion is especially poor in alkaline inks.
In addition, in most thermal ink jet heads the resin material adheres to in different regions an inorganic layer such as silicon nitride or silicon oxide. In other regions the resin is adhering to a tantalum layer used for cavitation protection. Such tantalum layer has a lower adhesion force than the silicon nitride layer to the resinous material constituting the flow path-forming member. Therefore the resin may peel off of the tantalum layer. In order to prevent this from occurring, Yabe in U.S. Pat. No. 6,676,241 discloses forming an adhesion layer composed of polyetheramide resin between the substrate and the flow path-forming member. In this case improved adhesion can be maintained between silicon nitride or Tantalum layer and adjoining flow path member resin. However it is important that this adhesion layer be properly patterned so that no portion is in contact with the electrothermal element. Patterning of this layer includes extra steps in the fabrication, increasing expense and lowering yield. Also since the resin constituting the flow path member is still in contact with the ink it could swell causing stresses to develop between it and the adhesion layer again causing delamination of the flow path member.
Stout et al., in U.S. Pat. No. 6,739,519 also discloses a method of monolithically fabricating an ink flow path and chamber with a nozzle plate using photodefinable epoxy over a sacrificial resist layer or alternatively, with a double exposure of a photodefinable epoxy. The patent discusses the problem of continued adhesion between the epoxy nozzle plate and the substrate. Since the epoxy has a much larger thermal coefficient of expansion than the substrate thermal stresses can develop during firing of the heaters leading to delamination. The patent proposes the use of a primer layer between nozzle plate and substrate. However the epoxy interface is still in close proximity to the heater.
The nozzle plate formed from a resin material is gas permeable. Therefore the ink in the chamber below the nozzle plate is subjected to increased evaporation. As a result, properties of the ink, such as viscosity, in the chamber may change causing degradation of ejection characteristics. Also, air from the outside entering the chamber can cause bubble formation again degrading the ejection. Inoue et al., in U.S. Pat. No. 6,186,616 discloses adding a metal layer to the top of the nozzle plate resin to prevent air ingestion. However care must be taken that good adhesion is formed between the resin and metal layer. Also the metal must be compatible with the ink so that it does not corrode. Higher temperature deposited materials cannot be used due to the thermal restrictions of the resin material.
With the inside of a chamber formed with epoxy another issue is the wetting of the chamber walls with the ink. It is important that the inner chamber walls be wetting with the ink. Otherwise priming of the head will be difficult. Also, after a drop is ejected the chamber is depleted of ink and must completely refill before another drop can be fired. Non-wetting walls will impede the refill process. The contact angle of the epoxy wall can be lowered, for example, by exposure to oxygen plasma. However the surface returns to a non-wetting state over time. Also the oxygen plasma roughens the surface of the epoxy that again impedes refill.
It would therefore be advantageous to have an alternative choice for the inner chamber wall that is wetting with the ink, such as silicon oxide or silicon nitride. Such layers have excellent adhesion to the substrate layers used in the printhead. These layers are deposited at high temperatures and have other excellent properties for use in contact with the ink such as; material robustness, low thermal expansion, low moisture absorption and moisture permeability,
Ramaswami et al., in U.S. Pat. No. 6,482,574 discloses an all-inorganic chamber by depositing a thick 5-20μm layer of oxide, patterning and etching to form the chamber, filling and planarizing a sacrificial layer, depositing a nozzle plate, and removing the sacrificial material. It is difficult to process such thick layers of oxide with long deposition and etch times. Such thick layers also have a tendency to crack due to stress build-up.
In commonly assigned U.S. Pat. No. 6,644,786 a chamber formation method is disclosed for a thermal actuator drop ejector. Non-photoimageable polyimide is patterned as the sacrificial layer allowing deposition of a high temperature inorganic structural layer such as silicon oxide or silicon nitride to form the chamber walls and nozzle plate. In this case only one deposition of the inorganic layer is needed to define both chamber walls and nozzle plate. Although this process eliminates the disadvantages of a polymer nozzle plate, a silicon oxide layer is a brittle material so that a printhead with chambers made this way can be more fragile. Also, thicker inorganic nozzle plates are harder to produce. Another aspect of an inorganic nozzle plate is that it is difficult to form a nozzle with a retrograde profile. A nozzle with a retrograde profile is advantageous for drop ejection stability and refill.
There is therefore a need for a chamber formation process that provides good adhesion to the substrate, an inner chamber material with good stability and wetting properties with respect to the ink, adjustable nozzle plate thickness, a nozzle with a retrograde profile, and a top surface, which is non-wetting with the ink.